Neuroanatomical experiments will be performed in order to obtain further information concerning the sources of axons traversing the hippocampal commissures in rats. These studies will involve pressure injection of nanoliter quantities of horseradish peroxidase (HRP) into subfield Ca4 (CA3c) of the hippocampus and into the polymorph area of the dentate gyrus. Retrograde transport of the HRP enzyme will allow us to identify the cell bodies of origin for axous projecting via the commissures into these areas. In addition electrophysiological monitoring of field potentials and a high-resolution computer graphics system will be used to facilitate precise positioning of the injecting micropipette in the extreme anterior and posterior extents of the hippocampal formation. Coordinates obtained from standard sterotaxic atlases are of little use in this regard as the neuronoal circuits of the hippocampus are not organized in the standard planes. These experiments are intended to extend current knowledge concerning the neuroanatomical ordering of cells projecting axons in the hippocampal commissures.